LLC resonant converters are widely employed in switching power supply due to the use of soft-switch technique, high efficiency and other advantages.
FIG. 1 illustrates a prior art half-bridge LLC resonant converter 50A. As shown in FIG. 1, the resonant converter 50A comprises a square-wave generator 501, a resonant network 502, an isolated transformer T, a rectifier network and a load. The square-wave generator 501 is illustrated as to have a half-bridge topology comprising a high-side switch M1 and a low-side switch M2 connected in series between a positive terminal and a negative terminal of a DC power supply VIN. Herein, the high-side switch M1 is controlled by a high-side control signal VG1, and the low-side switch M2 is controlled by a low-side control signal VG2 which has the same duty cycle as the high-side control signal VG1. Ideally, this duty cycle is 0.5. The square-wave generator 501 converts the DC power supply VIN to a square-wave signal VSW by controlling the high-side switch M1 and the low-side switch M2 to switch on and off in a complementary manner.
The resonant network 502 is illustrated as a LLC resonant network having a resonant inductor Lr, a resonant inductor Lm and a resonant capacitor Cr, wherein the resonant inductor Lm is connected in parallel with a primary winding of the isolated transformer T. Generally, the resonant inductor Lm is a field winding. The resonant network 502 converts the square-wave signal VSW to an AC (Alternating Current) voltage signal.
The rectifier network is coupled between a secondary winding of the isolated transformer T and the load. The rectifier network converts the AC voltage signal to an output voltage signal VOUT.
The half-bridge LLC resonant converter 50A further includes a control circuit comprising a voltage sensing circuit, a current sensing circuit, a mode judging circuit and a frequency controller. The voltage sensing circuit senses the output voltage signal VOUT to generate a feedback voltage signal VFB which is indicative of the output voltage signal VOUT. The current sensing circuit senses a current Ir flowing through the resonant inductor Lr to generate a current sense signal VCS. Usually, the current sense signal VCS is a voltage signal which is indicative of the current Ir. The mode judging circuit receives the current sense signal VCS, and compares the current sense signal VCS with a threshold to generate a mode signal MC which is used to indicate whether the half-bridge LLC resonant converter 50A operates in an inductive mode or a capacitive mode. The frequency controller receives the feedback voltage signal VFB and the mode signal MC, and generates the high-side control signal VG1 and the low-side control signal VG2 based on the feedback voltage signal VFB and the mode signal MC.
The LLC resonant converter 50A regulates the output voltage signal VOUT by changing its switching frequency, i.e., changing the switching frequency of the high-side switch M1 and the switching frequency of the low-side switch M2, wherein the switching frequency of the high-side switch M1 is the same as the switching frequency of the low-side switch M2.
As can be appreciated, the LLC resonant converter 50A is able to operate in the capacitive mode or the inductive mode depending on its switching frequency. Generally, in order to realize a function of Zero Voltage Switching (ZVS), the LLC resonant converter 50A should be controlled to operate in the inductive mode. If the LLC resonant converter 50A enters into the capacitive mode, it cannot realize the function of ZVS, which can cause the high-side switch M1 and the low-side switch M2 to be damaged.
FIG. 2 illustrates schematic waveform diagrams 50B of the resonant converter 50A in the inductive mode. As shown in FIG. 2, the current Ir lags behind the square wave signal VSW. Thus, the resonant converter 50A works in the inductive mode, in which the high-side switch M1 and the low-side switch M2 can be turned on at a zero voltage stress.
Generally, when the resonant converter 50A enters into the capacitive mode, the frequency controller of the control circuit will increase the switching frequency of the resonant converter 50A for exiting the capacitive mode. FIG. 3 illustrates a prior art frequency controller 50C, and FIG. 4 illustrates an operation waveform diagram 50D of the LLC resonant converter 50A controlled by the prior art frequency controller 50C of FIG. 3. Referring to FIG. 3 and FIG. 4, when the resonant converter 50A enters into the capacitive mode, a soft-start capacitor CSS will be discharged through turning a soft-start switch MSS on in response to the mode signal MC. Subsequently, an increasing rate and a decreasing rate of a setting voltage VCT across a setting capacitor CT are increased so that the switching frequency of the resonant converter 50A is increased. See FIG. 4, the switching cycle of the resonant converter 50A is decreased from T0 to T1 after the soft-start switch MSS is turned on.
Although increasing the switching frequency of the resonant converter 50A as described above can make the resonant converter 50A return to the inductive mode from the capacitive mode, it is still desired to improve the response speed of the resonant converter 50A for returning back to the inductive mode once it enters into the capacitive mode.